An internal combustion engine is used as a drive for motor vehicles. Some engines may include a turbocharger to boost the engine in order to allow for a smaller displacement engine. Liquid cooling systems are of major relevance in connection with boosted internal combustion engines. For example, the endeavor to achieve as close-fitting a packaging of the engine and turbocharger as possible basically result in higher thermal loading upon the internal combustion engine, in particular upon individual components and assemblies.
The cylinder head of a turbocharged internal combustion engine is subjected to greater thermal stress than the cylinder head of naturally aspirated engine because of the higher exhaust gas temperatures produced as a result of the turbocharger.
In order to implement as close-fitting a packaging as possible in the engine space, the aim is to have a compact type of construction, where, it is considered expedient to bring together the exhaust gas lines for discharging the exhaust gases so as to form an exhaust manifold inside the cylinder head, that is to say to integrate the manifold in the cylinder head. However, a cylinder head designed in this way is subjected to higher thermal loads than a conventional cylinder head equipped with an external manifold and therefore presents increased cooling requirements.
In order to minimize fuel consumption, in addition to the development of consumption-optimized combustion methods, measures for weight reduction are in this case at the forefront. The use of alternative materials is also expedient, where the aluminum preferably used, for example, for cylinder heads leads to a marked weight reduction, but is less capable of withstanding thermal load. This leads to an increased cooling demand and therefore to increased cooling requirements.
The heat released during combustion as a result of the exothermal chemical combustion of the fuel is discharged partially via the walls on the cylinder head which delimit the combustion space, and partially, via the exhaust gas stream, to the adjacent components and into the surroundings. In order to keep the thermal load upon the cylinder head within limits, part of the heat flow introduced into the cylinder head has to be extracted from the cylinder head again. The heat quantity discharged into the surroundings from the surface of the internal combustion engine by radiation and heat conduction is not adequate for efficient cooling, and therefore the cylinder head is often equipped with liquid cooling with which cooling inside the cylinder head is brought about by means of forced convection.
Liquid cooling results in the cylinder head being equipped with a coolant jacket, that is to say the arrangement of coolant ducts carrying the coolant through the cylinder head, thus making the cylinder head construction have a complex structure. For this case, on the one hand, the strength of the mechanically and thermally highly loaded cylinder head is weakened by the coolant ducts being introduced. On the other hand, unlike air cooling, the heat has to be conducted first to the cylinder head surface in order to be discharged. The heat is transferred, even inside the cylinder head, to the coolant, usually water mixed with additives. The coolant is in this case conveyed by means of a pump arranged in the cooling circuit, so that it circulates in the coolant jacket. The heat transferred to the coolant is thereby discharged from inside the cylinder head and is extracted from the coolant again in a heat exchanger.
However, even a liquid-cooled cylinder head may overheat. Thus, the cooling of the cylinder head described in EP 1 722 090 A2 proves inadequate in practice, and, particularly in the region where the exhaust gas lines converge into one common exhaust gas line, thermal overloading may occur which can be reflected, for example, in the form of material fusions.
In order to prevent this, in an internal combustion engine equipped with a cylinder head according to EP 1 722 090 A2, an enrichment (λ<1) is carried out whenever high exhaust gas temperatures are to be expected. In this case, more fuel is injected than can be burnt by means of the air quantity provided, the additional fuel likewise being heated and evaporated, so that the temperature of the combustion gases falls. However, this procedure is considered a disadvantage in energy terms, particularly with regard to the fuel consumption of the internal combustion engine, and with regard to pollutant emissions. In particular, the necessary enrichment may not make it possible to operate the internal combustion engine, as would be optimal, for example, for an exhaust gas retreatment system provided.
Overheating may become noticeable in that the coolant located in the coolant jacket evaporates in places. In the places where the coolant evaporates, a thin gas layer is formed which covers the inner wall of the coolant jacket, that is to say the boundary wall, and greatly reduces the heat transfer at this location. The wall material lying beneath the layer of gaseous coolant may overheat and fuse. Furthermore, gas bubbles formed may abruptly implode, if the vapor pressure is overshot or the temperature decreases. The latter leads to material damage similar to that resulting from cavitation. Moreover, overheating also impairs the properties of the coolant, that is to say its ability to cool or to absorb heat. The phenomena described above occur in thermally highly loaded regions of the boundary wall which is arranged between an exhaust gas-carrying line and the coolant jacket.
Additionally, turbocharger turbines provided in the engine may be liquid cooled. So that more cost-effective materials can be used for producing the turbine, the turbine may be equipped with liquid cooling which greatly reduces the thermal load upon the turbine or turbine casing by the hot exhaust gases and consequently allows the use of materials less capable of withstanding thermal load.
To implement cooling, the turbine casing is often provided with at least one coolant jacket. The casing may be is a casting and the coolant jacket formed during the casting operation as an integral part of a monolithic casing, the casing may be constructed in a modular manner, and a cavity, which serves as a coolant jacket, formed during assembly.
A turbine configured according to the last-mentioned concept is described, for example, in German laid-open publication DE 10 2008 011 257 A1. Liquid cooling of the turbine is implemented in that the actual turbine casing is provided with cladding, so as to form between the casing and the at least one cladding element arranged at a distance a cavity into which coolant can be introduced. The casing extended by the cladding then comprises the coolant jacket, and it is therefore also considered as the casing of the turbine in the context of the present disclosure. EP 1 384 857 A2 likewise discloses a turbine, the casing of which is equipped with a coolant jacket which is acted upon with seawater. The turbine casing is a casting formed in one piece.
What has been said with regard to overheating in connection with the cylinder head also applies in a similar way to the turbine casing, such that traditional liquid cooling systems of turbines may be subject to local regions of thermal overload resulting in overheating and possible damage to the turbine.
The inventors have recognized the issues with the above approaches and offer a system herein to at least partly address them. In one embodiment, an internal combustion engine is provided. The engine comprises at least one exhaust gas line, at least one coolant jacket, and a common boundary wall separating the at least one exhaust gas line and the at least one coolant jacket, wherein the common boundary wall includes a surface structure provided on sides of the coolant jacket in at least one locally limited region.
In this way, the coolant jacket has a boundary wall which, in contrast to previous boundary walls, is not designed to be even, but, instead, to be deliberately uneven in places, in that a surface structure is introduced into the wall on the coolant side. By a surface structure being introduced, the area available for heat transfer is increased. Moreover, the velocity near the wall rises since the surface structure generates turbulences. The two effects improve, that is to say intensify, the heat transfer. The introduction of heat from the wall into the coolant and therefore the cooling capacity increase.
In another embodiment, a system for reducing thermal loading comprises a cylinder head including a plurality of exhaust lines, the plurality of exhaust lines merging together in one or more confluence regions, an exhaust manifold integrated into the cylinder head and coupled to the plurality of exhaust lines, a coolant jacket integrated in the cylinder head and separated from the plurality of exhaust lines by one or more boundary walls, and at least one element positioned only on sides of the one or more boundary walls that face into the coolant jacket, the at least one element located only in the one or more confluence regions.
If appropriate, as a result of improved cooling, an enrichment of the fuel/air mixture with the aim of lowering the exhaust gas temperature may be dispensed with. This proves advantageous particularly with regard to the fuel consumption and the emission behavior of the internal combustion engine. Furthermore, more freedom in controlling the internal combustion engine arises, since possible enrichment for lowering the exhaust gas temperature in the context of engine control is avoidable.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.